1. Field of the Invention
The present invention relates to a gypsum board used in building construction and to a process for its manufacture; and more particularly, to a non-woven glass fiber mat comprising a blend of glass fibers having different diameters and lengths bonded together with a resinous latex binder, a gypsum board or similar cementitious product in panel form faced on at least one side with such a mat, and processes for the manufacture thereof.
2. Description of the Prior Art
Construction boards formed of a gypsum core sandwiched between facing layers are used in the construction of virtually every modern building. Various forms of such construction boards, generally known as gypsum boards, are employed as a surface for walls and ceilings and the like, both interior and exterior. Other forms are used in exterior finishing and insulation systems, interior lath systems, and roofing systems. All of these forms are relatively easy and inexpensive to install, finish, and maintain In suitable forms, they are relatively fire resistant.
Although paper-faced gypsum wallboard is most commonly used for finishing interior walls and ceilings, other forms with different kinds of facings have superior properties that are essential for other uses. One known facing material is non-woven fiberglass mat.
Gypsum wallboard and gypsum panels are traditionally manufactured by a continuous process. A gypsum slurry is first generated in a mechanical mixer (sometimes called a pin mixer) by mixing at least one of anhydrous calcium sulfate (CaSO4) and calcium sulfate hemihydrate (CaSO4.½H2O, also known as calcined gypsum), water, and other substances, which may include set accelerants, waterproofing agents, mineral, glass, or other synthetic reinforcing fibers, and the like. The gypsum slurry is normally deposited on a continuously advancing, lower facing sheet, such as kraft paper or a non-woven fibrous mat. Various additives, e.g. cellulose and glass fibers, are often added to the slurry to strengthen the gypsum core once it is dry or set. Starch is frequently added to the slurry in order to improve the adhesion between the gypsum core and the facing. Foam may be added to reduce the density of the slurry and the resulting set gypsum core. A continuously advancing upper facing sheet is laid over the gypsum and the edges of the upper and lower facing sheets are pasted to each other with a suitable adhesive. The facing sheets and gypsum slurry are passed between parallel upper and lower forming plates or rolls in order to generate an integrated and continuous flat strip of unset gypsum sandwiched between the sheets. Such a flat strip of unset gypsum is known as a facing or liner. The strip is conveyed over a series of continuous moving belts and rollers for a period of several minutes, during which time the core begins to hydrate back to gypsum (CaSO4.2H2O). The process is conventionally termed “setting,” since the rehydrated gypsum is relatively hard. During each transfer between belts and/or rolls, the strip is stressed in a way that can cause the facing to delaminate from the gypsum core if its adhesion is not sufficient. Once the gypsum core has set sufficiently, the continuous strip is cut into shorter lengths or even individual boards or panels of prescribed length. The set core is generally termed a gypsum core, notwithstanding the presence of other constituents and reinforcements, such as those delineated above. Preferably, the set core comprises at least 85% by weight of hydrated gypsum.
After the cutting step, the gypsum boards are fed into drying ovens or kilns so as to evaporate excess water. Inside the drying ovens, the boards are blown with hot drying air. After the dried gypsum boards are removed from the ovens, the ends of the boards are trimmed off and the boards are cut to desired sizes. The boards are commonly sold to the building industry in the form of sheets nominally 4 feet wide and 8 to 12 feet or more long and in thicknesses from nominally about ¼ to 1 inches, the width and length dimensions defining the two large faces of the board.
While paper is widely used as a facing material for gypsum board products because of its low cost, many applications demand water resistance that paper facing cannot provide. Upon exposure to water either directly in liquid form or indirectly through exposure to high humidity, paper is highly prone to degradation, such as by delamination, that substantially compromises its mechanical strength. Gypsum products typically rely on the integrity of the facing as a major contributor to their structural strength. Consequently, paper-faced products are generally not suited for use in either exterior applications or for interior locations in which exposure to high moisture or humidity is presumed. Alternative products have employed water resistant additives to the gypsum core itself or use of non-paper facers on which a further water resistant coating is added. These expedients are not always sufficient to provide the needed properties, and they often entail higher weight and complicated and costly additional manufacturing steps.
In addition, there is growing attention being given to the issue of mold and mildew growth in building interiors and the potential adverse health impact such activity might have on building occupants. The paper facing of conventional gypsum board contains wood pulp and other organic materials that may act in the presence of moisture or high humidity as nutrients for such microbial growth. A satisfactory alternative facing material less susceptible to growth is highly sought.
A further drawback of paper-faced gypsum board is flame resistance. In a building fire, the exposed paper facing quickly burns away. Although the gypsum itself is not flammable, once the facing is gone the board's mechanical strength is greatly impaired. At some stage thereafter the board is highly likely to collapse, permitting fire to spread to the underlying framing members and adjacent areas of a building, with obvious and serious consequences. A board having a facing less susceptible to burning would at least survive longer in a fire and thus be highly desirable in protecting both people and property.
In an attempt to overcome these and other problems, a number of alternatives to paper facing have been proposed. U.S. Pat. No. 4,647,496 discloses an exterior insulation system including a fibrous mat-faced gypsum board having a set gypsum core that is water-resistant. The fibrous mat is preferably sufficiently porous for the water in the gypsum slurry to evaporate during the production drying operation as the gypsum sets. The mat comprises fibrous material that can be either mineral-type or a synthetic resin. One preferred mat comprises non-woven glass fibers, randomly oriented and secured together with a modified or plasticized urea formaldehyde resin binder, and sold as DURA-GLASS® 7502 by the Manville Building Materials Corporation.
However, gypsum board products incorporating such conventional fibrous mats have proven to have certain drawbacks. While fibrous mats are undesirably more costly than the traditionally used kraft paper, there are other, more troublesome issues as well. Some persons are found to be quite sensitive to the fiberglass mat, and develop skin irritations and abrasions when exposed to the mat at various stages, including the initial production of the mat, the manufacture of composite gypsum board with the mat facing, and during the cutting, handling, and fastening operations (e.g., with nails or screws) that attend installation of the end product during building construction. Handling of the mat, and especially cutting, is believed to release glass fibers responsible for the irritation. The fibers may either become airborne or be transferred by direct contact. As a result, workers are generally forced to wear long-sleeved shirts and long pants and to use protective equipment such as dust masks. Such measures are especially unpleasant in the sweaty, hot and humid conditions often encountered either in manufacturing facilities or on a construction jobsite.
Many of the known glass-fiber faced gypsum boards also suffer from their relatively rough surfaces. For some applications, such as external insulation systems, a smooth trowel coating of stucco or similar material must be applied to provide sufficient water resistance. A rough surface necessarily entails a coating that requires substantial amounts of material applied in a relatively thick layer to attain adequate coverage and a smooth surface.
Known glass fiber mat systems in many cases also lack strength and resistance to mat delamination. The formaldehyde-based binders often used are being intensively scrutinized as having possible health risks, particularly when used in interior products.
There have been suggestions that a small portion of the glass fiber in such mats be replaced by polymer fiber materials and that an acrylic binder be used instead of urea formaldehyde resin. While gypsum boards incorporating such mats have somewhat improved strength and handling characteristics, they are undesirably more expensive to make and stiffer and less fire resistant. Moreover, the problems of irritation from dust released, e.g. during cutting, remain.
Another form of mat-faced gypsum board is known from U.S. Pat. No. 4,879,173, which discloses a mat of non-woven fibers having a reinforcing resinous binder that can comprise a single resin or a mixture of resins, either thermoplastic or thermosetting. Exemplary resins disclosed include a styrene-acrylic copolymer and a self-crosslinking vinyl acetate-acrylic copolymer. A small amount of the binder is applied to the surface of the mat and penetrates but part of the way therethrough. The board is said to be useful as a support member in a built-up roof. The highly textured surface of the mat binder provides many interstices into which can flow an adhesive used to adhere an overlying component. However, considerable care is required in using a mat containing substantial numbers of voids as a facer for gypsum board. Conventional processing that incorporates deposition of a relatively wet slurry is generally found to result in considerable intrusion of the slurry through the mat and onto the faced surface, which is frequently undesirable. Prevention of this excess intrusion typically requires very careful control of the slurry viscosity, which, in turn, frequently leads to other production problems. Alternative mats, which inherently limit intrusion, yet still have sufficient permeability to permit water to escape during the formation and heat drying of the gypsum board, are thus eagerly sought as a simpler alternative.
A fibrous mat facer with improved bleedthrough resistance and useful as a facer substrate or carrier for receiving a curable substance in a fluid state is disclosed by U.S. Pat. No. 4,637,951. The porous, non-woven mat comprises a blend of microfibers intermixed and dispersed with base fibers and bound with a binder comprising a water miscible combination of a heat settable polymer. The mat is said to be useful in forming composite materials employing a curable thermoset, preferably foamable material such as a polyurethane or polyisocyanurate rigid foam board and as a carrier web in the vinyl flooring industry where the settable polymer comprises a vinyl plastisol. However, mat bound with a thermoset binder has been found to have relatively low delamination strength.
U.S. Pat. No. 5,883,024 to O'Haver-Smith et al. provides a fibrous mat-faced gypsum board said to exhibit improved resistance to skin irritation and itching. The result is achieved by incorporating a minor portion by weight, preferably from about 5 to 25% by weight, of organic fibers. The benefit of reduced skin irritation is not achieved with less than 5% organic fibers. The binders used are preferably acrylic or PVC-based. The '024 patent further discloses the use of a secondary reinforcing binder that also may be termed a secondary coating. This secondary binder preferably imparts resistant to water, heat, and alkalinity. In order to inhibit bleed-through of gypsum through the facer of the '024 construction board, control of the viscosity of the gypsum slurry is suggested, e.g. by incorporation of a viscosity control agent, such as paper fiber, cellulose thickeners, bentonite clays, and starches.
Still another mat-faced gypsum board is disclosed by U.S. Pat. No. 6,001,496 to O'Haver-Smith. The mat employs inorganic fibers having a diameter of less than about 15 μm and has a basis weight of greater than about 1.85 lb/100 ft2. As a result of gypsum slurry bleed-through, mats formed from fibers having diameters predominantly of about 16 μm or greater are said not to fully satisfy the objects of the invention, even if the mats have a relatively high basis weight, e.g. a basis weight of 2.1 lb/100 ft2 or greater. The '496 patent further discloses a gypsum board production process in which a gypsum slurry, sandwiched between two fibrous mats of the foregoing type, is passed through an extrusion wedge to exert a compressive force on the work product and thereby improve the uniformity of the thickness of the finished board.
Notwithstanding the advances in the field of gypsum boards and related articles, there remains a need for a readily and inexpensively produced mat-faced gypsum board having one or more of a smoother surface, a stronger internal bond to prevent delamination of the facer when subjected to prolonged wetness after installation, and better flame and mold resistance.